Hydrogen is the most abundant element in the Universe: 91% by numbers of atoms, 70% by mass. All this hydrogen is primeval in origin, created in the earliest phase of the Universe (see Big Bang theory). The amount of deuterium formed relative to the amount of hydrogen would have been very sensitive to the density of matter at the time of formation: deuterium readily combines with an additional neutron to form tritium, which rapidly decays into an isotope of helium. Thus if the Universe was very dense in its first few minutes, most of the deuterium would have been converted to helium. Deuterium is not easy to detect but recent measurements of the ratio of deuterium to hydrogen give an upper limit of about 2 × 10–5 for interstellar gas.
Hydrogen can exist in atomic, molecular, and ionized forms. Ionized hydrogen, H+, more usually denoted H II, results when neutral atoms are stripped of their electrons. It occurs at high temperatures, as in the centers of stars and in H II regions. Nuclear fusion of hydrogen ions, i.e. protons, in the stellar core generates the energy of main-sequence stars by either the proton-proton chain reaction or the carbon cycle. Ionized hydrogen can also be produced by photoionization, as in emission nebulae, and may then be detected by its recombination line emission.
In addition to the positive ion, the negative ion, H–, can occur when an electron attaches itself loosely to a neutral atom. A continuous spectrum of radiation is emitted in the process. The negative ion is not very stable and breaks up easily with the second electron escaping with any amount of energy: this results in a continuous absorption spectrum. These two processes, producing continuous emission and absorption of both light and infrared radiation, take place concurrently, as happens in the Sun's photosphere. The H2– ion has recently been discovered.
Neutral hydrogen, usually denoted H I, occurs throughout interstellar space as filaments and clouds (H I regions) of varying density. It is detected by means of the 21-cm hydrogen line emission. At low temperatures and when the hydrogen density is sufficiently high, pairs of hydrogen atoms can combine to form molecular hydrogen, H2, which exists in discrete molecular clouds. The molecular ion H3 + forms in such clouds when a molecule, H2, is ionized and then further reacts with H2. The molecular ion is thought to have an important role in interstellar chemistry, allowing protons to be transferred to oxygen, carbon, and other heavy atoms, thereby leading to the formation of many different interstellar molecules. Molecular hydrogen is also a major constituent of the atmosphere of the giant planets, with liquid hydrogen forming the bulk of the interiors of Jupiter and Saturn.
Hydrogen can be observed at a number of wavelengths including radio (21 cm), infrared, visible, and ultraviolet wavelengths (see hydrogen spectrum).